The present invention is generally related to cellular radiotelephones and systems, and more particularly to an international mobile station identification method and signaling system for cellular radiotelephones and systems.
Cellular radiotelephones (referred to as mobile stations or MSs) currently in use in the Americas have a ten-digit mobile identification number ("MS number") that incudes a three-digit area code ("MIN2") and a seven-digit local number ("MIN1"). Each ten-digit MS number can be duplicated in the USA, Brazil, Argentina, Chile and other Americas countries. In landline telephone systems, the telephone numbers in each country further include a country code that is unique to each country and appended to the landline telephone number. Cellular radiotelephone systems have adopted a similar numbering plan by appending five-digits consisting of a mobile country code ("MCC") and a mobile network code ("MNC") to the ten-digit MS number. Therefore, for purposes of international mobile station identification ("IMSI"), each MS has an international MS number consisting of an MCC, MNC, MIN2 and MIN1.
However, existing American-type cellular systems have been designed to only use MIN2 and MIN1. As a result, an IMSI MS from Argentina may not be allowed operate in the USA since it will very likely have the same MIN2 and MIN1 as that of an old MS in the USA. For example, the first word ("Word A") of an order message directed to an MS contains MIN1, the local MS number, and has no additional bits. As shown in Table 1 below, MIN1 is a twenty-four bit field included in Word 1 of a message.
TABLE 1 ______________________________________ Word 1 - Abbreviated Address Word LENGTH FIELD (BITS) ______________________________________ T1T2 2 DCC 2 MIN1.sub.23-0 24 P 12 ______________________________________
Since MIN1 can be identical for MSs with different MCCs, a page from a BS directed to an IMSI MS could result in a false page to a old MS with the same MIN1.
In order to implement IMSI in the Americas in new MSs, it is necessary to include the MCC and MNC with each MS number to provide an IMSI MS number. Any solution to this problem must be backwards compatible such that old MSs listening to the cellular radiotelephone system control channel will not falsely interpret a page to an IMSI MS. One possibility is to add additional words to the pages sent by cellular base stations ("BSs") to the MSs on the forward control channel. However, that solution is impractical since forward channel capacity is wasted by the need to deliver the two additional IMSI messages for each page or order message, and cellular radiotelephone systems in the Americas are reaching capacity limits in some service areas due to the increasing number of new MSs being added each day.
Additional complications due to this solution are encountered on the reverse control channel carrying messages or orders from an IMSI MS to a BS. An additional word can not be accommodated in an origination from an MS. Also, responses from an IMSI MS must not be interpreted by an old-type BS that can not accommodate IMSI, and therefore a new message type would be needed for each access message type from and IMSI MS. However, there are not enough available unassigned message types in existing cellular radiotelephone systems in the Americas to accommodate this. For the foregoing reasons, there is a need for an new IMSI process which is backwards compatible with existing cellular radiotelephone systems and that can be implemented without degradation in cellular radiotelephone system capacity.